Twit of a girl who lost her head (3)
I believe the answer is:
ass
'twit' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ass that I've seen before include "Braying animal" , "Small hoofed animal" , "Donkey or silly person" , "Bottom, temporarily" , "Foolish person; animal" .)